The former Republican governor made the announcement in an op-ed piece published in Sunday's Tampa Bay Times. The endorsement came as Republicans are gathering in the Tampa Bay area for the GOP convention.

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Crist wrote that he's confident Obama is the right leader for the country.

"I applaud and share his vision of a future built by a strong and confident middle class in an economy that gives us the opportunity to reap prosperity through hard work and personal responsibility," Crist wrote.

Republicans, on the eve of their national convention, were quick to disparage Crist's endorsement.

GOP Chairman Lenny Curry called the move an attempt by Crist "to shed his skin for a political comeback."

"Despite the threat Florida is facing from a severe storm, Charlie Crist has demonstrated, yet again, that his political ambition will always come before the needs of Floridians."

Crist, who was a Republican when he was elected governor in 2006, is currently registered with no party affiliation. He left the Republican Party during his unsuccessful bid for a U.S. Senate seat in 2010. Marco Rubio, a Republican from Miami, defeated Crist in that election.

In the op-ed piece Crist noted that an element of the Republican Party has leaned to the far right on many issues important to women, immigrants, seniors and students.

"The truth is that the party has failed to demonstrate the kind of leadership or seriousness voters deserve."

Curry noted that Crist had previously disagreed with Obama on numerous issues.

Crist has been a politician for most of his adult life. He won a state Senate seat in 1992 before making an unsuccessful 1998 U.S. Senate bid. He then won three straight statewide offices — education commissioner in 2000, attorney general in 2002 and governor in 2006.